Motor

ABSTRACT

A motor includes a rotating portion with a shaft extending along a rotation axis and a rotor that rotates together with the shaft, a stationary portion with a stator located radially outside of the rotor and a magnetic sensor located above the rotor to detect a rotational position of the rotor. The rotor includes a rotor core, a rotor magnet located radially outside of the rotor core and under the magnetic sensor, in contact with the rotor core or located opposite to the rotor core with a first gap therebetween. A recessed portion is recessed from each of an upper end surface and an outer circumferential surface of the rotor core. An inner circumferential surface of a lower end portion of the rotor holder is fixed to a radially inner surface of the recessed portion.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a motor.

2. Description of the Related Art

In a so-called inner-rotor motor, in which a rotating shaft and a rotormagnet are arranged radially inside of an armature, a bending of theshaft tends to occur more easily with an increasing rotational speed ofthe motor. A bending of the shaft may cause vibrations and noise of themotor as a whole. In recent years, due to an increase in the rotationalspeed of motors, there has been a demand for preventing a bending of theshaft. A known inner-rotor motor is described, for example, in JP-A2013-99094.

In the motor described in JP-A 2013-99094, a shaft and a rotor magnetare fixed to each other through a sleeve and a rotor yoke (see paragraph[0019] and FIG. 1 of JP-A 2013-99094). Each of the sleeve and the rotoryoke is fixed to the shaft at one axial position. With this structure,it is difficult to improve rigidity of the shaft and its vicinity, andit is also difficult to prevent a bending of the shaft while the motoris running.

An effective way to prevent a bending of the shaft is to arrange a rotorcore around the shaft.

In the motor described in JP-A 2013-99094, the rotor magnet is arrangedto project toward a circuit board relative to a stator core (see FIG. 1of JP-A 2013-99094). In the case where the circuit board has mountedthereon magnetic sensors used to detect the circumferential position ofthe rotor magnet, it is necessary to ensure that the magnetic sensorsare able to detect the circumferential position of the rotor magnet withsufficient precision. This requires some a special configuration to beused, such as reducing the distance between the circuit board and therotor magnet as in the motor described in JP-A 2013-99094.

However, extending the rotor core up to an end portion of the rotormagnet on a side closer to the circuit board, which would not contributeto improving a torque characteristic of the motor, would result in anundesirable increased weight of the motor, resulting in a deteriorationin the torque characteristic of the motor.

SUMMARY OF THE INVENTION

A motor according to a preferred embodiment of the present inventionincludes a stationary portion and a rotating portion that rotates abouta rotation axis extending in a vertical direction. The rotating portionincludes a shaft extending along the rotation axis, and a rotor thatrotates together with the shaft. The stationary portion includes astator located radially outside of the rotor, and a magnetic sensorlocated above the rotor to detect a rotational position of the rotor.The rotor includes a rotor core including a ferromagnetic body and fixedto the shaft, a rotor magnet located radially outside of the rotor coreand under the magnetic sensor, in contact with the rotor core or locatedopposite to the rotor core with a first gap intervening therebetween,and including an upper end surface positioned at an axial level higherthan an axial level of an upper end surface of the rotor core, and atleast one rotor holder including a ferromagnetic body and located abovethe rotor core and radially inside of the rotor magnet. The rotor coreis defined by laminated steel sheets placed one upon another in an axialdirection. The rotor core includes a recessed portion recessed from eachof the upper end surface and an outer circumferential surface of therotor core. An inner circumferential surface of a lower end portion ofthe at least one rotor holder is fixed to a radially inner surface ofthe recessed portion.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a motor according to a firstpreferred embodiment of the present invention.

FIG. 2 is a cross-sectional view of a motor according to a secondpreferred embodiment of the present invention.

FIG. 3 is an exploded perspective view of a rotating portion accordingto the second preferred embodiment of the present invention.

FIG. 4 is a partial cross-sectional view of the motor according to thesecond preferred embodiment of the present invention.

FIG. 5 is a partial cross-sectional view of a motor according to amodification of the second preferred embodiment of the presentinvention.

FIG. 6 is a partial cross-sectional view of a motor according to amodification of the second preferred embodiment of the presentinvention.

FIG. 7 is a partial cross-sectional view of a motor according to amodification of the second preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. It is assumedherein that a direction parallel or substantially parallel to a rotationaxis of a motor is referred to by term “axial direction”, “axial”, or“axially”, that directions perpendicular or substantially perpendicularto the rotation axis of the motor are referred to by the term “radialdirection”, “radial”, or “radially”, and that a direction along acircular arc centered on the rotation axis of the motor is referred toby the term “circumferential direction”, “circumferential”, or“circumferentially”. It is also assumed herein that an axial directionis a vertical direction, and that a side on which a magnetic sensor islocated with respect to a rotor is defined as an upper side. The shapeof each member or portion and relative positions of different members orportions will be described based on the above assumptions. It should benoted, however, that the above definitions of the vertical direction andthe upper and lower sides are made simply for the sake of convenience indescription, and should not be construed to restrict in any way theorientation of a motor according to any preferred embodiment of thepresent invention when in use.

First, a motor 1A according to a preferred embodiment of the presentinvention will be described below with reference to FIG. 1. FIG. 1 is across-sectional view of the motor 1A. As illustrated in FIG. 1, themotor 1A includes a stationary portion 2A and a rotating portion 3A.

The stationary portion 2A preferably includes a stator 23A and magneticsensors 241A. The stator 23A is an armature located radially outside ofa rotor 32A of the rotating portion 3A. The rotor 32A will be describedbelow. The magnetic sensors 241A are located above the rotor 32Adescribed below to detect the rotational position of the rotor 32A.Specifically, the magnetic sensors 241A are arranged over a rotor magnet322A described below. The magnetic sensors 241A are configured to detectthe circumferential position of the rotor magnet 322A by detectingmagnetic flux passing from the rotor magnet 322A toward each magneticsensor 241A.

The rotating portion 3A is supported to be rotatable about a rotationaxis 9A with respect to the stationary portion 2A. The rotating portion3A is thus configured to rotate about the rotation axis 9A. The rotatingportion 3A preferably includes a shaft 31A and the rotor 32A.

The shaft 31A is a columnar member extending along the rotation axis 9A.The rotor 32A is configured to rotate together with the shaft 31A. Therotor 32A preferably includes a rotor core 321A, the rotor magnet 322A,and a rotor holder 323A.

The rotor core 321A is a tubular member that is a ferromagnetic bodyfixed to the shaft 31A. Specifically, the rotor core 321A is preferablydefined by laminated steel sheets, i.e., a plurality of steel sheetsplaced one upon another in the axial direction. The rotor core 321A ispositioned around the shaft 31A to increase rigidity of the rotor 32Aincluding the shaft 31A. This contributes to preventing a bending of theshaft 31A while the motor 1A is running. This in turn contributes toreducing vibrations and noise of the motor 1A.

The rotor magnet 322A is an annular magnet. An outer circumferentialsurface of the rotor magnet 322A is located radially opposite to aninner circumferential surface of the stator core 231A. The outercircumferential surface of the rotor magnet 322A includes north andsouth poles arranged to alternate with each other in a circumferentialdirection. Note that a plurality of magnets may be used in place of therotor magnet 322A. The rotor magnet 322A is positioned under themagnetic sensors 241A.

The rotor magnet 322A is located radially outside of the rotor core321A. According to the present preferred embodiment, the rotor magnet322A and the rotor core 321A are preferably in contact with each other.That is, a radial distance from the rotation axis 9A to an innercircumferential surface of the rotor magnet 322A and a radial distancefrom the rotation axis 9A to an outer circumferential surface of therotor core 321A are preferably equal or substantially equal to eachother.

In the case where the rotor magnet 322A and the rotor core 321A areradially opposite to each other with a gap intervening therebetween,this gap is referred to as a first gap. In this case, the first gap isnarrow enough to allow the rotor core 321A to define and function as aback yoke for the rotor magnet 322A.

An upper end surface of the rotor magnet 322A is positioned at an axiallevel higher than an axial level of an upper end surface of the rotorcore 321A. Thus, the axial distance between the rotor magnet 322A andeach magnetic sensor 241A is reduced. This enables the magnetic sensors241A to detect the circumferential position of the rotor magnet 322Awith higher precision.

The rotor holder 323A is located above the rotor core 321A and radiallyinside of the rotor magnet 322A. The rotor holder 323A is aferromagnetic body. Moreover, the rotor holder 323A preferably includesa cylindrical or substantially cylindrical outer cylindrical portion 61Aextending along the rotation axis 9A. The outer cylindrical portion 61Amay be either in contact with the rotor magnet 322A or located radiallyopposite to the rotor magnet 322A with a gap intervening therebetween.In the case where the outer cylindrical portion 61A and the rotor magnet322A are located opposite to each other with the gap interveningtherebetween, this gap is referred to as a second gap. In this case, thesecond gap is narrow enough to allow the outer cylindrical portion 61Ato define and function as a back yoke for the rotor magnet 322A.

Thus, above the rotor core 321A, the rotor holder 323A defines andfunctions as the back yoke for the rotor magnet 322A. Consequently, evenin the case where the rotor magnet 322A projects upward above the upperend surface of the rotor core 321A, magnetic flux is reduced only to alimited degree at a projecting portion of the rotor magnet 322A.Accordingly, a reduction in the magnetic flux passing from the rotormagnet 322A toward each magnetic sensor 241A is reduced or eliminated.This contributes to further improving the precision with which thecircumferential position of the rotor magnet 322A is detected by themagnetic sensors 241A.

Moreover, since the rotor holder 323A defines and functions as the backyoke, it is not necessary to extend the rotor core 321A up to a vicinityof an upper end portion of the rotor magnet 322A. This reduces anincrease in the weight of the motor 1A due to the weight of the rotorcore 321A. As described above, the motor 1A according to the presentpreferred embodiment is able to reduce the possibility of a bending ofthe shaft 31A, and also to achieve improved precision with which thecircumferential position of the rotor magnet 322A is detected by themagnetic sensors 241A while reducing an increase in the weight of themotor 1A.

Next, a motor 1 according to another preferred embodiment of the presentinvention will now be described below with reference to FIGS. 2, 3, and4. FIG. 2 is a cross-sectional view of the motor 1. FIG. 3 is anexploded perspective view of a rotating portion 3 of the motor 1. FIG. 4is a partial cross-sectional view of the motor 1. The motor 1 ispreferably used, for example, in a blower to provide rotational power torotate an impeller of the blower.

As illustrated in FIG. 2, the motor 1 according to the present preferredembodiment includes a stationary portion 2, the rotating portion 3, anda bearing portion 4.

The stationary portion 2 according to the present preferred embodimentpreferably includes a housing 21, a cap 22, a stator 23, and a circuitboard 24.

The housing 21 preferably includes a housing bottom portion 211, ahousing tubular portion 212, and a first bearing holding portion 213.The housing bottom portion 211 is preferably perpendicular orsubstantially perpendicular to a rotation axis 9. The housing tubularportion 212 is tubular, and extends axially upward from the housingbottom portion 211 along the rotation axis 9. The first bearing holdingportion 213 projects downward from the housing bottom portion 211. Anouter circumferential surface of a lower ball bearing 41, which will bedescribed below, of the bearing portion 4 is fixed inside the firstbearing holding portion 213.

The cap 22 preferably includes a cap bottom portion 221, a cap tubularportion 222, and a second bearing holding portion 223. The cap bottomportion 221 is perpendicular or substantially perpendicular to therotation axis 9. The cap tubular portion 222 is preferably tubular, andextends axially downward from the cap bottom portion 221 along therotation axis 9. The second bearing holding portion 223 projects upwardfrom the cap bottom portion 221. An outer circumferential surface of anupper ball bearing 42, which will be described below, of the bearingportion 4 is fixed inside the second bearing holding portion 223.

The stator 23, at least a portion of the circuit board 24, and a rotor32 of the rotating portion 3 are accommodated inside a case defined bythe housing 21 and the cap 22. The rotor 32 will be described below.Each of the housing 21 and the cap 22 is preferably made of, forexample, a galvanized steel sheet, SUS, or the like. The housing 21 andthe cap 22 according to the present preferred embodiment are preferablymade of the same material. Note that the housing 21 and the cap 22 maybe made of different materials.

The stator 23 is preferably an armature defined by a stator core 231, aninsulator 232, and coils 233. The stator 23 is located radially outsideof the rotor 32.

The stator core 231 is preferably defined by electromagnetic steelsheets placed one upon another in an axial direction. The stator core231 includes an annular core back 51 and a plurality of teeth 52 whichproject radially inward from the core back 51. An outer circumferentialsurface of the core back 51 is fixed to an inner circumferential surfaceof the housing tubular portion 212. The plurality of teeth 52 arepreferably positioned at regular or substantially regular intervals in acircumferential direction.

The insulator 232 is preferably a member made of, for example, a resinand arranged to cover a portion of a surface of the stator core 231. Theinsulator 232 according to the present preferred embodiment includes acover portion 53 configured to cover an upper end surface of the statorcore 231, and a board support portion 54 which projects upward from thecover portion 53.

Each of the coils 233 is wound around a separate one of the teeth 52with the insulator 232 intervening therebetween. Each coil 233 isdefined by a conducting wire wound around a corresponding one of theteeth 52 with the insulator 232 intervening therebetween.

The circuit board 24 is configured to supply electric drive currents tothe coils 233 of the stator 23. The circuit board 24 according to thepresent preferred embodiment is provided in a space enclosed by thehousing 21 and the cap 22. The circuit board 24 is fixed to the boardsupport portion 54 of the insulator 232.

Magnetic sensors 241 located above the rotor 32 to detect the rotationalposition of the rotor 32 are preferably located on a lower surface ofthe circuit board 24. The magnetic sensors 241 detect the rotationalposition of the rotor 32 by detecting the circumferential position of arotor magnet 322 described below. Each magnetic sensor 241 ispreferably, for example, a Hall element.

According to the present preferred embodiment, the number of magneticsensors 241 is preferably three, and the three magnetic sensors 241 arepreferably spaced from one another by sixty degrees about the rotationaxis 9, for example. Thus, the magnetic sensors 241 detect thecircumferential position of the rotor magnet 322, and perform feedbackto enable appropriate drive control of the motor 1. To accurately detectthe circumferential position of the rotor magnet 322, it is desirablethat the axial distance between an upper end surface of the rotor magnet322 and each magnetic sensor 241 be as short as possible.

The cap tubular portion 222 of the cap 22 according to the presentpreferred embodiment preferably includes a cutout 220 passing throughthe cap tubular portion 222 in a radial direction. A portion of a leadwire 242 connected to the circuit board 24 is preferably arranged withinthe cutout 220. Note that a portion of the circuit board 24 or aconnector may also be arranged in the cutout 220 if so desired. Alsonote that the cap 22 may include a hole passing through the cap tubularportion 222 in a radial direction in place of the cutout 220. Thus, thecircuit board 24 is electrically connected to an external device throughthe cutout 220 to enable electric drive currents necessary to drive themotor 1 to be supplied to the circuit board 24.

The rotating portion 3 is supported to be rotatable about the rotationaxis 9 with respect to the stationary portion 2. The rotating portion 3according to the present preferred embodiment includes a shaft 31 andthe rotor 32.

As illustrated in FIGS. 2 and 3, the shaft 31 is a columnar memberextending along the rotation axis 9. A metal, such as, for example,stainless steel, is preferably used as a material of the shaft 31. Theshaft 31 is configured to rotate about the rotation axis 9 while beingsupported by the bearing portion 4. A lower end portion of the shaft 31projects downward below the housing 21. The lower end portion of theshaft 31 is preferably fixed to an object which is to be driven by themotor 1, such as, for example, the impeller of the blower.

An outer circumferential surface of the shaft 31 includes a press fitportion 311 to which a rotor core 321 described below is press fitted.The press fit portion 311 is preferably, for example, knurled to fix therotor core 321 thereto with increased strength. The press fit portion311 is knurled in a linear pattern with straight ridges extending alongthe rotation axis 9 according to the present preferred embodiment, butmay alternatively be knurled in a crisscross pattern, e.g., a diamondpattern or a square or rectangular pattern, or in a diagonal pattern.Also note that knurling may not necessarily be applied to the shaft 31.

As illustrated in FIG. 2, the rotor 32 preferably includes the rotorcore 321, the rotor magnet 322, a first rotor holder 323, and a secondrotor holder 324. The rotor 32 is configured to rotate together with theshaft 31.

As illustrated in FIGS. 2 and 3, the rotor core 321 is cylindrical andis defined by electromagnetic steel sheets placed one upon another inthe axial direction. That is, the rotor core 321 is a ferromagneticbody. An inner circumferential surface of the rotor core 321 is fixed tothe shaft 31 through press fitting. The rotor core 321 is positionedaround the shaft 31 to increase rigidity of the shaft 31. Thiscontributes to preventing a bending of the shaft 31 while the motor 1 isrunning. This in turn contributes to reducing and preventing vibrationsand noise of the motor 1.

According to the present preferred embodiment, the stator core 231 hasan axial dimension equal or substantially equal to the axial dimensionof the rotor core 321. Accordingly, the steel sheets which define thestator core 231 and the steel sheets which define the rotor core 321 canpreferably be stamped from the same steel sheet. Specifically, the steelsheets of the rotor core 321 are stamped out from portions of the steelsheet which are radially inside of portions of the steel sheet fromwhich the steel sheets of the stator core 231 are stamped out. Thus,material costs of the stator core 231 and the rotor core 321 arereduced, and scraps are also reduced.

The rotor magnet 322 is an annular magnet centered on the rotation axis9. The rotor magnet 322 is located radially outside of the rotor core321 and under the magnetic sensors 241, and is configured to rotatetogether with the shaft 31.

An outer circumferential surface of the rotor magnet 322 is locatedradially opposite to an inner end of each of the plurality of teeth 52of the stator core 231. In addition, the outer circumferential surfaceof the rotor magnet 322 includes north and south poles arranged toalternate with each other in the circumferential direction. Note that,in place of the annular rotor magnet 322, a plurality of magnets may beused. In this case, the plurality of magnets are arranged in thecircumferential direction such that north and south pole surfacesalternate with each other.

As illustrated in FIG. 4, an inner circumferential surface of the rotormagnet 322 is located radially opposite to the rotor core 321 with afirst gap 71 intervening therebetween. The first gap 71 is narrow enoughto allow the rotor core 321 to define and function as a back yoke forthe rotor magnet 322. This contributes to increasing magnetic flux whichpasses from the rotor magnet 322 toward the stator core 231 at aposition opposed to the stator core 231. Thus, a torque characteristicof the motor 1 is improved.

According to the present preferred embodiment, an outer circumferentialsurface of the rotor core 321 and the inner circumferential surface ofthe rotor magnet 322 are preferably fixed to each other through, forexample, adhesion. Thus, an adhesive P is arranged in the first gap 71.

The upper end surface of the rotor magnet 322 is positioned at an axiallevel higher than an axial level of an upper end surface of the rotorcore 321. Thus, the axial distance between the rotor magnet 322 and eachmagnetic sensor 241 is preferably reduced. This enables the magneticsensors 241 to detect the circumferential position of the rotor magnet322 with higher precision.

In addition, according to the present preferred embodiment, a lower endsurface of the rotor magnet 322 is positioned at an axial level lowerthan an axial level of a lower end surface of the rotor core 321.Moreover, the axial dimension of a portion of the rotor magnet 322 whichis above the upper end surface of the rotor core 321 is preferably equalor substantially equal to the axial dimension of a portion of the rotormagnet 322 which is below the lower end surface of the rotor core 321.Accordingly, magnetic flux originating from the rotor magnet 322 isdistributed in a balanced manner above and below an axial middle of thestator 23.

Each of the first rotor holder 323 and the second rotor holder 324 ispreferably made of a ferromagnetic material, such as, for example,steel. Each of the first rotor holder 323 and the second rotor holder324 according to the present preferred embodiment is preferably astamping, i.e., an article obtained by subjecting a sheet material topress working. Thus, a production cost of each of the first rotor holder323 and the second rotor holder 324 is reduced compared to the casewhere each of the first rotor holder 323 and the second rotor holder 324is manufactured by another method, such as, for example, laminatingsteel sheets.

The first rotor holder 323 is located above the rotor core 321. Thesecond rotor holder 324 is located below the rotor core 321. Inaddition, each of the first rotor holder 323 and the second rotor holder324 is preferably located radially inside of the rotor magnet 322. Thesecond rotor holder 324 preferably has the same or substantially thesame shape as that of the first rotor holder 323, and the first andsecond rotor holders 323 and 324 are arranged upside down relative toeach other. That is, the first rotor holder 323 and the second rotorholder 324 are preferably symmetric or substantially symmetric withrespect to a horizontal plane. Thus, the magnetic flux originating fromthe rotor magnet 322 is distributed in a more balanced manner above andbelow the axial middle of the stator 23. The second rotor holder 324 issimilar to the first rotor holder 323 in the shape, dimensions, and soon, and descriptions thereof are therefore omitted.

The first rotor holder 323 preferably includes an outer cylindricalportion 61, an inner cylindrical portion 62, and an annular plateportion 63. Each of the outer cylindrical portion 61 and the innercylindrical portion 62 is a cylindrical or substantially cylindricalportion extending along the rotation axis 9. The outer cylindricalportion 61 extends along the inner circumferential surface of the rotormagnet 322.

In addition, as illustrated in FIG. 4, the outer cylindrical portion 61according to the present preferred embodiment is preferably locatedradially opposite to the inner circumferential surface of the rotormagnet 322 with a second gap 72 intervening therebetween. The second gap72 is narrow enough to allow the outer cylindrical portion 61 tofunction as a back yoke for the rotor magnet 322. According to thepresent preferred embodiment, the inner circumferential surface of therotor magnet 322 and an outer circumferential surface of the first rotorholder 323 are preferably fixed to each other through adhesion.Therefore, the adhesive P is arranged in the second gap 72.

Thus, above the rotor core 321, the first rotor holder 323 defines andfunctions as the back yoke for the rotor magnet 322. Consequently, evenin the case where the rotor magnet 322 projects upward above the upperend surface of the rotor core 321, magnetic flux is reduced only to alimited degree at a projecting portion of the rotor magnet 322.Accordingly, a reduction in the magnetic flux passing from the rotormagnet 322 toward each magnetic sensor 241 is reduced. This contributesto further improving the precision with which the circumferentialposition of the rotor magnet 322 is detected by the magnetic sensors241.

Here, the radial width of the first gap 71 corresponds to a differencebetween a radial distance r1 from the rotation axis 9 to the innercircumferential surface of the rotor magnet 322 and a radial distance r2from the rotation axis 9 to the outer circumferential surface of therotor core 321. Meanwhile, the radial width of the second gap 72corresponds to a difference between the radial distance r1 from therotation axis 9 to the inner circumferential surface of the rotor magnet322 and a radial distance r3 from the rotation axis 9 to the outercircumferential surface of the first rotor holder 323.

According to the present preferred embodiment, the radial distance r3from the rotation axis 9 to the outer circumferential surface of thefirst rotor holder 323 is equal to or smaller than the radial distancer2 from the rotation axis 9 to the outer circumferential surface of therotor core 321. That is, the radial width of the first gap 71 is smallerthan the radial width of the second gap 72.

As described above, the rotor core 321 is preferably defined bylaminating the stamped electromagnetic steel sheets in the axialdirection. Meanwhile, the first rotor holder 323 is preferably molded bypress working. Therefore, a radial dimensional accuracy of the rotorcore 321 is higher than a radial dimensional accuracy of the first rotorholder 323. Accordingly, the radial width of the first gap 71 ispreferably smaller than the radial width of the second gap 72 toincrease accuracy with which the rotor magnet 322 is positioned withrespect to the rotation axis 9. As a result, the rotor magnet 322 ismore easily arranged to be coaxial with the rotation axis 9.

In addition, since the radial width of the first gap 71 is smaller thanthe radial width of the second gap 72, the rotor core 321 defines andfunctions as the back yoke for the rotor magnet 322 more effectivelythan the first rotor holder 323. This contributes to further increasingmagnetic flux which passes from the rotor magnet 322 toward the statorcore 231 at the position opposed to the stator core 231. As a result,the torque characteristic of the motor 1 is further improved.

According to the present preferred embodiment, the axial position of theupper end surface of the rotor magnet 322 and the axial position of anupper end surface of the outer cylindrical portion 61 of the first rotorholder 323 are identical or substantially identical to each other. Thatis, the axial position of the upper end surface of the rotor magnet 322and the axial position of an upper end surface of a portion of the firstrotor holder 323 which is adjacent to the rotor magnet 322 are identicalor substantially identical to each other. This contributes to furtherincreasing magnetic flux passing from a vicinity of an upper end portionof the rotor magnet 322 toward each magnetic sensor 241. This enablesthe magnetic sensors 241 to detect the circumferential position of therotor magnet 322 with still higher precision.

The annular plate portion 63 is a portion which joins a lower endportion of the outer cylindrical portion 61 and a lower end portion ofthe inner cylindrical portion 62 to each other. According to the presentpreferred embodiment, a gap is preferably defined between a lower endsurface of the first rotor holder 323, that is, a lower end surface ofthe annular plate portion 63, and the upper end surface of the rotorcore 321. Thus, the rotor core 321 and the first rotor holder 323 do notrestrict the axial position of each other. Accordingly, a variation inthe axial dimension of the rotor core 321 or of the first rotor holder323 would preferably not prevent the rotor core 321 and the first rotorholder 323 from being positioned at their respective appropriate axialpositions. This allows, for example, the position of an axial middle ofthe rotor core 321, the position of an axial middle of the rotor magnet322, and the position of an axial midpoint between the first rotorholder 323 and the second rotor holder 324 to coincide with one another.

The inner cylindrical portion 62 is fixed to the shaft 31. The firstrotor holder 323 is fixed to the shaft 31 as a result of an innercircumferential surface of the inner cylindrical portion 62 of the firstrotor holder 323 being, for example, press fitted to the outercircumferential surface of the shaft 31. Thus, a radially inner endportion of the first rotor holder 323 is in contact with the outercircumferential surface of the shaft 31. This makes it easy to arrangethe shaft 31 and the first rotor holder 323 to be coaxial with eachother. Note that the second rotor holder 324 is also fixed to the shaft31 through press fitting in a manner similar to that in which the firstrotor holder 323 is fixed to the shaft 31.

In addition, since the first rotor holder 323 is molded by pressworking, a tapered portion which becomes radially more distant from therotor magnet 322 with decreasing distance from an axial end portionthereof is defined at a junction of the annular plate portion 63 and theouter cylindrical portion 61. Therefore, if the annular plate portion 63were joined to an upper end portion of the outer cylindrical portion 61,the radial width of the second gap 72 would increase at a positionradially inside the upper end portion of the rotor magnet 322. Thiswould make the function of the first rotor holder 323 as the back yokefor the vicinity of the upper end portion of the rotor magnet 322 lesseffective.

According to the present preferred embodiment, the annular plate portion63 is preferably joined to the lower end portion of the outercylindrical portion 61 to allow the radial distance between the outercylindrical portion 61 and the rotor magnet 322 to be uniform orsubstantially uniform in the vicinity of the upper end portion of theouter cylindrical portion 61. This contributes to preventing thefunction of the first rotor holder 323 as the back yoke for the vicinityof the upper end portion of the rotor magnet 322 from becoming lesseffective. This in turn contributes to preventing a reduction in theprecision with which the circumferential position of the rotor magnet322 is detected by the magnetic sensors 241.

The bearing portion 4 includes the lower ball bearing 41 and the upperball bearing 42. As described above, the outer circumferential surfacesof the lower ball bearing 41 and the upper ball bearing 42 are fixed tothe housing 21 and the cap 22, respectively. In addition, an innercircumferential surface of each of the lower ball bearing 41 and theupper ball bearing 42 is fixed to the outer circumferential surface ofthe shaft 31. The bearing portion 4 is thus configured to rotatablysupport the shaft 31. Note that, although the bearing portion 4according to the present preferred embodiment is defined by ballbearings, this is not essential to the present invention. The bearingportion 4 may alternatively be defined by a bearing mechanism of anothertype, such as, for example, a plain bearing or a fluid bearing.

Once the electric drive currents are supplied to the coils 233 throughthe circuit board 24, radial magnetic flux is generated around each ofthe teeth 52 of the stator core 231. Then, a circumferential torque isproduced by interaction between the magnetic flux of the teeth 52 andmagnetic flux of the rotor magnet 322, so that the rotating portion 3 iscaused to rotate about the rotation axis 9 with respect to thestationary portion 2.

As described above, in the motor 1, the rotor core 321 is arrangedaround the shaft 31 to prevent a bending of the shaft 31. Moreover,since the first rotor holder 323 defines and functions as the back yoke,it is not necessary to extend the rotor core 321 up to the vicinity ofthe upper end portion of the rotor magnet 322. This contributes toimproving the precision with which the circumferential position of therotor magnet 322 is detected by the magnetic sensors 241 while reducingan increase in the weight of the motor 1.

While preferred embodiments of the present invention have been describedabove, it is to be understood that the present invention is not limitedto the above-described preferred embodiments.

FIG. 5 is a partial cross-sectional view of a motor 1B according to amodification of the above-described preferred embodiment. In themodification illustrated in FIG. 5, a radial distance r2 from a rotationaxis 9B to an outer circumferential surface of a rotor core 321Bpreferably is equal to or smaller than a radial distance r3 from therotation axis 9B to an outer circumferential surface of a first rotorholder 323B.

Here, the radial width of a first gap 71B corresponds to a differencebetween a radial distance r1 from the rotation axis 9B to an innercircumferential surface of a rotor magnet 322B and the radial distancer2 from the rotation axis 9B to the outer circumferential surface of therotor core 321B. Meanwhile, the radial width of a second gap 72Bcorresponds to a difference between the radial distance r1 from therotation axis 9B to the inner circumferential surface of the rotormagnet 322B and the radial distance r3 from the rotation axis 9B to theouter circumferential surface of the first rotor holder 323B.

Thus, the radial width of the second gap 72B, which is a radial gapbetween the rotor magnet 322B and the first rotor holder 323B,preferably is equal to or smaller than the radial width of the first gap71B, which is a radial gap between the rotor magnet 322B and the rotorcore 321B.

When the radial width of the second gap 72B is reduced as in themodification illustrated in FIG. 5, a function of the first rotor holder323B as a back yoke for a vicinity of an upper end portion of the rotormagnet 322B is improved. This enables magnetic sensors 241B to detectthe circumferential position of the rotor magnet 322B with higherprecision.

FIG. 6 is a partial cross-sectional view of a motor 1C according toanother modification of the above-described preferred embodiment. In themodification illustrated in FIG. 6, a first rotor holder 323C is acylindrical member extending along a rotation axis 9C. In other words,the first rotor holder 323C includes only an outer cylindrical portion61C located opposite to a rotor magnet 322C with a second gap 72Cintervening therebetween.

In addition, a rotor core 321C includes a recessed portion 320C recessedfrom each of an upper end surface and an outer circumferential surfaceof the rotor core 321C. An inner circumferential surface of a lower endportion of the first rotor holder 323C is press fitted to a radiallyinner surface of the recessed portion 320C, such that the first rotorholder 323C is fixed to the rotor core 321C.

In the modification illustrated in FIG. 6, it is easy to define therotor core 321C and the first rotor holder 323C such that a radialdistance r2 from the rotation axis 9C to the outer circumferentialsurface of the rotor core 321C and a radial distance r3 from therotation axis 9C to an outer circumferential surface of the first rotorholder 323C are equal or substantially equal to each other.

When the radial distances r2 and r3 are equal or substantially equal toeach other, the radial width of a first gap 71C, which is a radial gapbetween the rotor magnet 322C and the rotor core 321C, and the radialwidth of the second gap 72C, which is a radial gap between the rotormagnet 322C and the first rotor holder 323C, are equal or substantiallyequal to each other. Thus, a back yoke is arranged such that a gapbetween an outer circumferential surface of the back yoke and an innercircumferential surface of the rotor magnet 322C has a uniform orsubstantially uniform width from an axially upper end to an axiallylower end thereof. This contributes to stabilizing magnetic flux passingfrom a surface of the rotor magnet 322C to a surrounding space.

FIG. 7 is a partial cross-sectional view of a motor 1D according toanother modification of the above-described preferred embodiment. In themodification illustrated in FIG. 7, a first rotor holder 323D ispreferably defined by laminated steel sheets, just as a rotor core 321Dis preferably defined by laminated steel sheets. The first rotor holder323D includes only an outer cylindrical portion 61D located opposite toa rotor magnet 322D with a second gap 72D intervening therebetween.

The steel sheet positioned at a lower end of the first rotor holder 323Dand the steel sheet positioned at an upper end of the rotor core 321Dare preferably fixed to each other by, for example, crimping or thelike, just as axially adjacent ones of the steel sheets within the rotorcore 321D are fixed to each other by crimping or the like.

In the modification illustrated in FIG. 7, each of the steel sheetswhich define the rotor core 321D and each of the steel sheets whichdefine the first rotor holder 323D are both molded by stamping.Accordingly, the rotor core 321D and the first rotor holder 323D areequivalent in radial dimensional accuracy. Therefore, it is easier todefine the rotor core 321D and the first rotor holder 323D such that aradial distance r2 from a rotation axis 9D to an outer circumferentialsurface of the rotor core 321D and a radial distance r3 from therotation axis 9D to an outer circumferential surface of the first rotorholder 323D are equal or substantially equal to each other than it is todefine the rotor core 321C and the first rotor holder 323C such that theradial distance r2 from the rotation axis 9C to the outercircumferential surface of the rotor core 321C and the radial distancer3 from the rotation axis 9C to the outer circumferential surface of thefirst rotor holder 323C are equal or substantially equal to each otherin the modification illustrated in FIG. 6.

When the radial distances r2 and r3 are more exactly equal to eachother, the radial width of a first gap 71D, which is a radial gapbetween the rotor magnet 322D and the rotor core 321D, and the radialwidth of the second gap 72D, which is a radial gap between the rotormagnet 322D and the first rotor holder 323D, are more exactly equal toeach other. Thus, a back yoke is configured such that a gap between anouter circumferential surface of the back yoke and an innercircumferential surface of the rotor magnet 322D has a uniform orsubstantially uniform width from an axially upper end to an axiallylower end thereof. This contributes to further stabilizing magnetic fluxpassing from a surface of the rotor magnet 322D to a surrounding space.

Although the rotor holder is fixed to the shaft in the above-describedpreferred embodiment, this is not essential to the present invention. Asin each of the modifications illustrated in FIGS. 6 and 7, the rotorholder may be fixed to the rotor core. Also note that the rotor holderand the rotor core may be fixed to each other by any other desirablefixing method, such as, for example, adhesion, welding, etc. Forexample, it may be so arranged that a raised portion and a recessedportion are defined in a lower end surface of the rotor holder and anupper end surface of the rotor core, respectively, and the raisedportion and the recessed portion are mated and crimped together to fixthe rotor holder and the rotor core to each other.

The motors according to the above-described preferred embodiments of thepresent invention are preferably used in a blower, for example. Note,however, that motors according to other preferred embodiments of thepresent invention may be used, for example, in office automationappliances, such as printers and copy machines, transportationequipment, such as automobiles, household electrical appliances, medicalappliances, disk drives, blower fans, and the like to generate a varietyof driving forces.

Features of the above-described preferred embodiments and themodifications thereof may be combined appropriately as long as noconflict arises.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

Preferred embodiments of the present invention are applicable to, forexample, motors.

What is claimed is:
 1. A motor comprising: a stationary portion; and arotating portion that rotates about a rotation axis extending in avertical direction; wherein the rotating portion includes: a shaftextending along the rotation axis; and a rotor that rotates togetherwith the shaft; the stationary portion includes: a stator locatedradially outside of the rotor; and a magnetic sensor located above therotor to detect a rotational position of the rotor; the rotor includes:a rotor core including a ferromagnetic body fixed to the shaft; a rotormagnet located radially outside of the rotor core and under the magneticsensor, in contact with the rotor core or located opposite to the rotorcore with a first gap intervening therebetween, and including an upperend surface located at an axial level higher than an axial level of anupper end surface of the rotor core; and at least one rotor holderincluding a ferromagnetic body, and located above the rotor core andradially inside of the rotor magnet; wherein the rotor core is definedby laminated steel sheets placed one upon another in an axial direction,and including a recessed portion recessed from each of the upper endsurface and an outer circumferential surface of the rotor core; and aninner circumferential surface of a lower end portion of the at least onerotor holder is fixed to a radially inner surface of the recessedportion.
 2. The motor according to claim 1, wherein a radial distancefrom the rotation axis to a radially inner surface of the rotor magnetand a radial distance from the rotation axis to a radially outer surfaceof the rotor core are equal or substantially equal to each other.
 3. Themotor according to claim 1, wherein the at least one rotor holder is acylindrical member extending along the rotation axis, and made of astamped material.
 4. The motor according to claim 3, wherein an innercircumferential surface of the rotor magnet is located radially oppositeto the rotor core with the first gap defined therebetween; the rotormagnet is located radially opposite to an inner circumferential surfaceof the rotor magnet with a second gap defined therebetween; and a radialwidth of the first gap is smaller than a radial width of the second gap.5. The motor according to claim 1, wherein a radial distance from therotation axis to the outer circumferential surface of the rotor core isequal or substantially equal to a radial distance from the rotation axisto an outer circumferential surface of the at least one rotor holder. 6.The motor according to claim 1, wherein an axial position of the upperend surface of the rotor magnet and an axial position of an upper endsurface of a portion of the at least one rotor holder which is adjacentto the rotor magnet are identical or substantially identical to eachother.
 7. The motor according to claim 1, wherein the stator includes: astator core defined by laminated steel sheets placed one upon another inthe axial direction; a plurality of coils attached to the stator core;and the stator core has an axial dimension equal or substantially equalto an axial dimension of the rotor core.
 8. The motor according to claim7, wherein a lower end surface of the rotor magnet is positioned at anaxial level lower than an axial level of a lower end surface of therotor core; and the at least one rotor holder includes: a first rotorholder located above the rotor core and radially inside of the rotormagnet; and a second rotor holder located below the rotor core andradially inside of the rotor magnet.
 9. The motor according to claim 8,wherein the rotor magnet is annular and is centered on the rotationaxis.
 10. The motor according to claim 9, wherein an outercircumferential surface of the shaft includes a knurled portion.
 11. Themotor according to claim 10, wherein an adhesive is provided between therotor core and the rotor magnet and also between the at least one rotorholder and the rotor magnet.